Hydrogen is the lightest, most abundant substance in the universe. Further, hydrogen is efficient in storing energy. For example, hydrogen has a heat of combustion per unit mass of 120.1 MJ/kg, which is about 3 times higher than the heat of combustion of gasoline.
Hydrogen can be utilized in environmentally friendly alternative energy conversion devices such as hydrogen combustion engines and fuel cells. In a hydrogen combustion engine, hydrogen can be combusted in the presence of oxygen to produce only water as a byproduct. Therefore, unlike gasoline, hydrogen can be combusted without producing environmentally unfriendly byproducts such as carbon dioxide, carbon monoxide, sulfur containing compounds and the like. In a fuel cell, hydrogen and oxygen can be catalyzed at opposite electrodes of the fuel cell to produce a potential difference across an electrochemical cell. As in hydrogen combustion engines, fuel cells can react hydrogen and oxygen to convert energy while producing water as the sole product.
Hydrogen is a very light gas and has a very low volumetric density at atmospheric pressure. Hydrogen can be produced and/or purified by many clean methods. For example, hydrogen can be produced from water through electrolysis powered by photoelectric cells. However, many challenges must be overcome to efficiently store hydrogen.
Several different strategies have been utilized to efficiently store hydrogen at high densities. A first strategy involves storing hydrogen at high pressure, for example, pressures up to 10,000 psig. However, storing hydrogen at a high pressure requires special structures such as, for example, hydrogen storage containers having very thick walls, which increases the overall weight and cost of the system. Further, high pressure hydrogen storage systems require costly valves, pumps, and control systems for maintaining hydrogen at high pressure levels and for transporting hydrogen between a high pressure environment for hydrogen storage and a low pressure environment for applications.
A second strategy for storing hydrogen is liquefying hydrogen at cryogenic temperatures. However, this strategy also has several drawbacks. For example, in order to liquefy hydrogen, extremely low temperature (that is, a temperature of about 20 Kelvin) is required. Therefore, storing liquid hydrogen is inefficient in that it requires high energy (approximately one-third the energy produced by hydrogen combustion) to obtain and maintain cryogenic storage temperatures and in that it requires an expensive refrigeration system.
A third strategy for storing hydrogen is hydrogen storage utilizing solid state hydrogen storage mediums, such as, for example, metal hydrides. Metal hydrides absorb and desorb hydrogen without refrigeration at low pressures that can be easily obtained. Therefore, metal hydrides do not have the high energy and high systems costs associated with high pressure hydrogen storage systems and liquid hydrogen storage systems. Further, hydrogen can be stored at a higher volumetric density in metal hydride storage systems than in either high pressure hydrogen storage systems or liquid hydrogen storage systems.
Currently, several challenges must be addressed to commercialize hydrogen storage systems utilizing solid hydrogen storage medium. Hydrogen molecules are very small, making hydrogen containment in a sealed container challenging. Further, the hydrogen storage container must be durable and must maintain its physical structure. So that it does not deform and rupture. Furthermore, the solid state hydrogen storage system must operate within acceptable safety margin. Further, the system must be easily manufacturable. Still further, the system must meet certain performance requirements. For example, the system must route hydrogen to specific locations within the system, must sufficiently control and maintain the temperature of hydrogen storage medium for adsorption and desorption of hydrogen, and must sufficiently maintain the hydrogen at selected elevated pressures. Still further, for vehicle applications, the system must store large amounts of hydrogen per unit weight and unit volume.
In addition to the stresses caused by storing hydrogen under pressures, hydrogen storage systems utilizing solid hydrogen storage medium are subjected to additional stresses caused by the expansion and contraction of hydrogen storage medium. For example, some hydrogen storage mediums expand by up to 25% during a hydrogen storage phase and then contract to its original volume during a hydrogen release phase. Unlike stresses associated with storing compressed gasses, systems utilizing solid hydrogen storage medium will produce localized stresses on the outer walls of the hydrogen storage container. These localized stresses are due to hydrogen storage medium expanding and producing stresses in contact areas between the material and the outer wall. Therefore, these localized pressures can be much greater than the hydrostatic pressure of hydrogen gas inside the storage chamber.
Other challenges that must be addressed related to efficient heat transfer and efficient hydrogen absorption and desorption rates. In order for this absorption process to occur, heat needs to be removed during the reaction process. In addition, heat needs to be supplied during the desorption process. Therefore, the system needs to be equipped to efficiently heat and cool the metal hydrides during hydrogen desorption and absorption. Further, in order to transfer hydrogen into and out of the system, hydrogen gas must be efficiently supplied and uniformly distributed to each of the hydrogen storage containers.
Examples of hydrogen storage systems are contained in U.S. Pat. Nos. 6,318,453, 6,709,497, 6,833,118, 6,969,545, 6,991,770, and 7,241,331. Each the systems disclosed in these patents utilize heat transfer elements inside a single chamber in which hydrogen storage medium is stored. These systems are heavy, bulky, difficult to manufacture. Some of these challenges were solved by the present inventors in U.S. Pat. No. 6,918,382. However, U.S. Pat. No. 6,918,382 differs from the instant disclosure several ways. For example, the current application teaches an improved heat transfer system over the air cooling apparatus of the '382 patent, Further, the '382 patent does not teach a hydrogen storage system with convenient hydrogen storage medium replacement means. Still further, the '382 system has straight tubular hydrogen storage modules that are connected via a complicated and costly external network of pipes and fittings. Still further, the '382 patent does not have force distribution means. Further differences of the present system are described throughout the disclosure.
In addition to challenges discussed above, hydrogen storage systems utilized in transportation applications must meet standards set by administrative agencies. For example, in the United States, the United States Department of Transportation has implemented requirements for systems for transporting hydrogen gas in vehicles. To achieve commercialization, solid state hydrogen storage systems must meet this standard.
Therefore, a need exists for a hydrogen storage system that solves the problems listed above and that can be utilized for vehicle or stationary hydrogen storage application and can be manufactured by large-scale manufacturing operations.